Process for cooling a silicon ingot having a vacancy dominated region to produce defect free silicon

ABSTRACT

A process for producing silicon which is substantially free of agglomerated intrinsic point defects in an ingot having a vacancy dominated region. An ingot is grown generally in accordance with the Czochralski method. While intrinsic point defects diffuse from or are annihilated within the ingot, at least a portion of the ingot is maintained above a temperature T A  at which intrinsic point defects agglomerate. The achievement of defect free silicon is thus substantially decoupled from process parameters, such as pull rate, and system parameters, such as axial temperature gradient in the ingot.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 10/035,540, filed Oct. 23, 2001, now U.S. Pat. No. 6,562,123,which is a continuation of U.S. patent application Ser. No. 09/344,709,filed Jun. 25, 1999, now U.S. Pat. No. 6,328,795, which claims priorityfrom U.S. Provisional Application Serial No. 60/090,723, filed Jun. 26,1998, U.S. Provisional Application Serial No. 60/104,087, filed Oct. 14,1998, and U.S. Provisional Application Serial No. 60/117,623 filed Jan.28, 1999.

BACKGROUND OF THE INVENTION

[0002] The present invention generally relates to the preparation ofsemiconductor grade single crystal silicon which is used in themanufacture of electronic components. More particularly, the presentinvention relates to a process for producing a single crystal siliconingot which is substantially devoid of agglomerated intrinsic pointdefects over the entire crystal radius and usable length of the ingots.

[0003] Single crystal silicon, which is the starting material for mostprocesses for the fabrication of semiconductor electronic components, iscommonly prepared by the so-called Czochralski (“Cz”) method. In thismethod, polycrystalline silicon (“polysilicon”) is charged to a crucibleand melted, a seed crystal is brought into contact with the moltensilicon and a single crystal is grown by slow extraction. Afterformation of a neck is complete, the diameter of the crystal is enlargedby decreasing the pulling rate and/or the melt temperature until thedesired or target diameter is reached. The cylindrical main body of thecrystal which has an approximately constant diameter is then grown bycontrolling the pull rate and the melt temperature while compensatingfor the decreasing melt level. Near the end of the growth process butbefore the crucible is emptied of molten silicon, the crystal diametermust be reduced gradually to form an end-cone. Typically, the end-coneis formed by increasing the crystal pull rate and heat supplied to thecrucible. When the diameter becomes small enough, the crystal is thenseparated from the melt.

[0004] In recent years, it has been recognized that a number of defectsin single crystal silicon form in the crystal growth chamber as thecrystal cools after solidification. Such defects arise, in part, due tothe presence of an excess (i.e., a concentration above the solubilitylimit) of intrinsic point defects in the crystal lattice, which arevacancies and self-interstitials. Silicon crystals grown from a melt aretypically grown with an excess of one or the other type of intrinsicpoint defect, either crystal lattice vacancies (“V”) or siliconself-interstitials (“I”). It has been suggested that the type andinitial concentration of these point defects in the silicon aredetermined at the time of solidification and, if these concentrationsreach a level of critical supersaturation in the system and the mobilityof the point defects is sufficiently high, a reaction, or anagglomeration event, will likely occur. Agglomerated intrinsic pointdefects in silicon can severely impact the yield potential of thematerial in the production of complex and highly integrated circuits.

[0005] Vacancy-type defects are recognized to be the origin of suchobservable crystal defects as D-defects, Flow Pattern Defects (FPDs),Gate Oxide Integrity (GOI) Defects, Crystal Originated Particle (COP)Defects, crystal originated Light Point Defects (LPDs), as well ascertain classes of bulk defects observed by infrared light scatteringtechniques such as Scanning Infrared Microscopy and Laser ScanningTomography. Also present in regions of excess vacancies are defectswhich act as the nuclei for ring oxidation induced stacking faults(OISF). It is speculated that this particular defect is a hightemperature nucleated oxygen agglomerate catalyzed by the presence ofexcess vacancies.

[0006] Defects relating to self-interstitials are less well studied.They are generally regarded as being low densities of interstitial-typedislocation loops or networks. Such defects are not responsible for gateoxide integrity failures, an important wafer performance criterion, butthey are widely recognized to be the cause of other types of devicefailures usually associated with current leakage problems.

[0007] The density of such vacancy and self-interstitial agglomerateddefects in Czochralski silicon is conventionally within the range ofabout 1*10³/cm³ to about 1*10⁷/cm³. While these values are relativelylow, agglomerated intrinsic point defects are of rapidly increasingimportance to device manufacturers and, in fact, are now seen asyield-limiting factors in device fabrication processes.

[0008] To date, there generally exists three main approaches to dealingwith the problem of agglomerated intrinsic point defects. The firstapproach includes methods which focus on crystal pulling techniques inorder to reduce the number density of agglomerated intrinsic pointdefects in the ingot. This approach can be further subdivided into thosemethods having crystal pulling conditions which result in the formationof vacancy dominated material, and those methods having crystal pullingconditions which result in the formation of self-interstitial dominatedmaterial. For example, it has been suggested that the number density ofagglomerated defects can be reduced by (i) controlling v/G,)to grow acrystal in which crystal lattice vacancies are the dominant intrinsicpoint defect, and (ii) influencing the nucleation rate of theagglomerated defects by altering (generally, by slowing down) thecooling rate of the silicon ingot from about 1100° C. to about 1050° C.during the crystal pulling process. While this approach reduces thenumber density of agglomerated defects, it does not prevent theirformation. As the requirements imposed by device manufacturers becomemore and more stringent, the presence of these defects will continue tobecome more of a problem.

[0009] Others have suggested reducing the pull rate, during the growthof the body of the crystal, to a value less than about 0.4 mm/minute.This suggestion, however, is also not satisfactory because such a slowpull rate leads to reduced throughput for each crystal puller. Moreimportantly, such pull rates lead to the formation of single crystalsilicon having a high concentration of self-interstitials. This highconcentration, in turn, leads to the formation of agglomeratedself-interstitial defects and all the resulting problems associated withsuch defects.

[0010] A second approach to dealing with the problem of agglomeratedintrinsic point defects includes methods which focus on the dissolutionor annihilation of agglomerated intrinsic point defects subsequent totheir formation. Generally, this is achieved by using high temperatureheat treatments of the silicon in wafer form. For example, Fusegawa etal. propose, in European Patent Application 503,816 A1, growing thesilicon ingot at a growth rate in excess of 0.8 mm/minute, and heattreating the wafers which are sliced from the ingot at a temperature inthe range of 1150° C. to 1280° C. to reduce the defect density in a thinregion near the wafer surface. The specific treatment needed will varydepending upon the concentration and location of agglomerated intrinsicpoint defects in the wafer. Different wafers cut from a crystal whichdoes not have a uniform axial concentration of such defects may requiredifferent post-growth processing conditions. Furthermore, such waferheat treatments are relatively costly, have the potential forintroducing metallic impurities into the silicon wafers, and are notuniversally effective for all types of crystal-related defects.

[0011] A third approach to dealing with the problem of agglomeratedintrinsic point defects is the epitaxial deposition of a thincrystalline layer of silicon on the surface of a single crystal siliconwafer. This process provides a single crystal silicon wafer having asurface which is substantially free of agglomerated intrinsic pointdefects. Epitaxial deposition, however, substantially increases the costof the wafer.

[0012] In view of these developments, a need continues to exist for amethod of single crystal silicon preparation which acts to prevent theformation of agglomerated intrinsic point defects by suppressing theagglomeration reactions which produce them. Rather than simply limitingthe rate at which such defects form, or attempting to annihilate some ofthe defects after they have formed, a method which acts to suppressagglomeration reactions would yield a silicon substrate that is free orsubstantially free of agglomerated intrinsic point defects. Such amethod would also afford single crystal silicon wafers having epi-likeyield potential, in terms of the number of integrated circuits obtainedper wafer, without having the high costs associated with an epitaxialprocess.

[0013] It is now recognized that silicon single crystal ingots can begrown which have virtually no defects produced by agglomeration ofintrinsic point defects. (See, e.g., PCT/US98/07365 and PCT/US98/07304.)A primary mechanism for the suppression of agglomeration reactions isthe radial out-diffusion of intrinsic point defects. If given sufficienttime at crystal temperatures in excess of the temperature T_(A) at whichagglomeration reactions will occur, self-interstitials and vacancieswill either combine and annihilate each other or diffuse to sinks on thesurface of the ingot.

[0014] Silicon self-interstitials appear to be extremely mobile attemperatures near the solidification temperature of silicon, i.e., about1410° C. This mobility, however, decreases as the temperature of thesingle crystal silicon ingot decreases. Generally, the diffusion rate ofself-interstitials slows such a considerable degree that they areessentially immobile for commercially practical time periods attemperatures less than about 700° C., and perhaps at temperatures asgreat as 800° C., 900° C., 1000° C., or even 1050° C.

[0015] It is to be noted in this regard that, although the temperatureat which a self-interstitial agglomeration reaction occurs may in theoryvary over a wide range of temperatures, as a practical matter this rangeappears to be relatively narrow for conventional, Czochralski-grownsilicon. This is a consequence of the relatively narrow range of initialself-interstitial concentrations which are typically obtained in silicongrown according to the Czochralski method. In general, therefore, aself-interstitial agglomeration reaction may occur, if at all, attemperatures (T_(A)) within the range of about 1100° C. to about 800°C., and typically at a temperature of about 1050° C.

[0016] By controlling the cooling rate of the ingot within a range oftemperatures in which self-interstitials appear to be mobile, theself-interstitials may be given more time to diffuse to sinks located atthe crystal surface, or to vacancy dominated regions, where they may beannihilated. The concentration of such interstitials may therefore besuppressed to a level low enough so that supersaturation ofself-interstitials (i.e., a concentration above the solubility limit)does not occur at a temperature at which the self-interstitials aresufficiently mobile to agglomerate. The same principles apply forsilicon vacancies. However, the relative immobility of the vacanciesmakes their outdiffusion more difficult.

[0017] It would be possible to produce single crystal ingots free ofagglomerated micro-defects in presently existing crystal pullers, butthere are a number of contradictory conditions present in the operationof the crystal puller and the ingot. Difficult compromises must be made,which materially impact the commercial practicality of the production ofdefect-free single crystal ingots. Growth of a single crystal siliconingot is schematically illustrated in FIG. 1. Silicon solidifies fromthe melt into the ingot at a temperature of about 1410° C. and isthereafter continuously cooled. At some location along the length of theingot L(T_(A)) above the melt surface the ingot will pass through anisotherm T_(A) at which agglomeration reactions occur (e.g., 1050° C.).The ingot will pass through this point during the time it is beinggrown.

[0018] Essentially, growing defect-free ingots would require thattemperature distribution in the hot zone be engineered to producesufficiently long residence times of the ingot at temperatures in excessof a temperature T_(A) (e.g., about 1050° C.) at which agglomerationreactions occur to permit the out-diffusion of the intrinsic pointdefects. Maximizing the residence time of axial segment of the ingotabove T_(A) requires that the pull rate be slowed. However, slowing thepull rate drastically reduces throughput for the crystal puller.

[0019] The required residence time of each axial segment of the ingot attemperatures in excess of T_(A) can be reduced somewhat by growing thecrystal so that self-interstitial intrinsic point defects predominate.Self-interstitial defects are substantially more mobile than vacancydefects. It is still necessary to minimize the initial concentration ofdefects. However, to minimize the number of defects, the pull rateshould be maximized within the interstitial growth conditions.

[0020] In order to produce a single crystal ingot which wassubstantially free of agglomerated micro-defects over its entire length,each axial segment along the full usable length of the ingot must passthrough T_(A) only after residing at temperature in excess of T_(A) fora time necessary to out-diffuse the intrinsic point defects. Thus, thesame relatively slow pull rate must be maintained even while theunusable end-cone of the ingot is being formed. Furthermore, the ingotmust be raised at the same slow rate even after it is formed so that thelower end of the usable constant diameter portion of the ingot hassufficient residence time at temperatures above T_(A).

[0021] The tension between pull rate and residence time necessary forout-diffusion of intrinsic point defects becomes more acute as thediameter of the crystal grown increases. As the diameter of the ingotincreases, the number of defects increases and the radial distancethrough which the defects must diffuse to the surface of the ingotincreases.

[0022] Still further, minimization of the time for out-diffusion ofseif-interstitials makes it desirable to minimize the radial variationin initial interstitial concentration. This is achieved by minimizingthe radial variation of the axial temperature gradient G₀(r). In orderto minimize the radial variation in the axial temperature gradient, itis desirable to minimize the average value of the axial temperaturegradient G,,at the ingot at the surface of the silicon melt. However inorder to maximize the pull rate which will achieve interstitial growthconditions, it is desirable to maximize the average value of G₀.

[0023] As a practical matter, very stringent process controls must bemaintained in the operation of the crystal puller to produce singlecrystal silicon ingots which are substantially free of agglomeratedintrinsic point defects. Moreover, there is a dramatic reduction inthroughput for the crystal puller. Thus, there is presently a need for aprocess to grow single crystal ingots free of agglomerated intrinsicpoint defects which decouples or substantially decouples the operationof the crystal puller from the conditions necessary to out-diffuseintrinsic point defects.

SUMMARY OF THE INVENTION

[0024] Among the several objects and features of the present inventionmay be noted the provision of a process of producing a single crystalsilicon ingot which is substantially free of agglomerated intrinsicpoint defects over the entire usable length of the ingot; the provisionof such a process which does not substantially diminish the throughputof the crystal puller; the provision of such a process whichsubstantially decouples the operating conditions of the crystal pullerfrom the conditions for production of the defect-free ingot; theprovision of such a process which substantially reduces the crystalpuller from limitations on pull rate for production of the defect-freeingot; and the provision of such a process which substantially reducesthe crystal puller from limitations on the average axial temperaturegradient G₀.

[0025] Briefly, therefore, the present invention is directed to aprocess for producing a single crystal silicon ingot having a seed-cone,an end-cone and a constant diameter portion between the seed-cone andend-cone. The ingot is grown from a silicon melt in accordance with theCzochralski method. Generally, the process comprises growing the ingotfrom the silicon melt and controlling the temperature of the ingot suchthat no portion of the ingot cools to a temperature less than atemperature T_(A) at which agglomeration of intrinsic point defects inthe ingot occurs during the time the ingot is being grown such that atleast the constant diameter portion of the ingot is substantially freeof agglomerated intrinsic point defects.

[0026] The present invention is further directed to a process forproducing a single crystal silicon ingot having a seed-cone, an end-coneand a constant diameter portion between the seed-cone and end-cone. Theingot is grown in a crystal puller from a silicon melt in accordancewith the Czochralski method. The crystal puller includes a lower growthchamber and an upper pulling chamber, and the process comprises loweringa seed crystal into contact with the silicon melt located in the growthchamber of the crystal puller and withdrawing the seed crystal from themelt so as to cause silicon from the melt to freeze for forming thesingle crystal silicon ingot. The fully formed ingot is pulled into thepulling chamber, the pulling chamber is then isolated from the growthchamber while the temperature in the pulling chamber is maintained abovea temperature T_(A) at which agglomeration of intrinsic point defects inthe ingot occurs.

[0027] Other objects and features of the present invention will be inpart apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a schematic view illustrating existing single crystalsilicon ingot growth showing the passage of the ingot during growththrough an isotherm at which agglomeration reactions occur;

[0029]FIG. 2 is a graph which shows an example of how the initialconcentration of self-interstitials [I] and vacancies [V] changes withthe value of v/G₀, where v is the pull rate and G₀ is the average axialtemperature gradient;

[0030]FIG. 3 is a schematic view of a crystal puller apparatus whichpermits removal of a pulling chamber portion of the crystal puller andreplacement with another pulling chamber;

[0031]FIG. 4 is a schematic view of a crystal puller apparatus whichpermits the ingot to be removed from the pulling chamber to a holdingchamber located to the side of the pulling chamber;

[0032]FIG. 5 is a schematic view of a crystal puller apparatus whichpermits the ingot to be removed from the pulling chamber to a holdingchamber located generally above the pulling chamber;

[0033]FIG. 6A is a graph of the normalized growth rate as a function ofcrystal length, as described in the Example;

[0034]FIG. 6B is a series of photographs of axial cuts of segments of aningot, ranging from the shoulder to where end-cone growth begins,following copper decoration and a defect-delineating etch, as describedin the Example; and,

[0035]FIG. 6C is a series of photographs of axial cuts of segments of aningot, ranging from the seed-cone to the end-cone, following copperdecoration and a defect-delineating etch, as described in the Example.

[0036] Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Referring now to the drawings, and in particular to FIG. 3,apparatus for carrying out a process according to the present inventionis schematically shown to include a crystal puller indicated generallyat 10. The crystal puller includes a growth chamber 12 which houses acrucible 14 for containing a silicon melt M. The crucible 14 is mountedfor rotation within the growth chamber 12 in the conventional manner.The crystal puller 10 also includes a conventional heater and insulation(not shown) for heating the silicon in the crucible 14 to produce andmaintain its molten state. The crystal puller 10 further comprises apull chamber 16 located on top of the growth chamber 12 and capable ofopening into the growth chamber for receiving a single crystal siliconingot I being grown from the molten silicon M. The pull chamber 16 has awinch mechanism 18 for raising and lowering a pull wire 20 having a seedcrystal chuck 22 at its end. Alternatively, a pulling mechanism (notshown) which employs a shaft rather than a pull wire may be used. Thechuck 22 carries a seed crystal (not shown) which is used to initiateformation of the ingot I, according to the Czochralski method.

[0038] The pull chamber 16 is equipped with a valve 24 for closing offthe pull chamber from the growth chamber 12. Similarly, the growthchamber 12 has its own valve 26 for closing itself off from the pullchamber 16. The pull chamber is releasably mounted on the growth chamber12 so that the entire pull chamber 16 can be removed from the growthchamber. The apparatus further includes another pull chamber 16′(corresponding parts of the other pull chamber 16′ will be designated bythe same reference numerals as the pull chamber 16 with the addition ofafter the numeral). The other pull chamber 16′ can be mounted on thegrowth chamber 12 and used to grow another ingot I′. However asillustrated in FIG. 3, one ingot I is being grown in the pull chamber 16while the other ingot I′ is held in the other pull chamber 16′ at alocation spaced away from the growth chamber 12.

[0039] A process carried out according to the principles of the presentinvention will now be described with reference to the apparatus shown inFIG. 3. As will be understood, the process may be carried out by otherapparatus (some examples of which are described hereinafter). Thecrystal puller 10 is initially prepared in a conventional way, such asby placing solid polysilicon in the crucible 14 and energizing theheater to melt the silicon forming the silicon melt M. The winchmechanism 18 is activated to let out the pull wire 20 and lower thechuck 22 so that the seed crystal contacts the surface of the melt. Thecrucible 14 and the pull wire 20 are both rotated about a vertical axis.As the seed crystal begins to melt, the winch mechanism 18 is activatedto slowly reel in the pull wire 20, raising the seed crystal from themelt. Silicon from the melt M freezes onto the seed crystal in amonocrystalline lattice, thereby beginning to form the ingot I.

[0040] The ingot initially has a seed cone SC, which has a diameterwhich increases to the point where it equals the diameter of the ingotwhich is desired to be grown (typically somewhat greater than thedesired diameter of semiconductor wafers ultimately formed from theingot). A constant diameter portion CD is grown by controlling the pullrate and heating of the ingot I. An end cone EC′ (shown only on theother ingot I′) is formed in order to separate the ingot I from the meltM when the constant diameter CD portion has reached the length needed.This length is limited by the geometry of the crystal puller 10. The endcone EC′ is also formed by controlling (i.e., generally increasing) thepull rate of the ingot and by the application of heat. After separationfrom the melt M, the ingot I is pulled entirely within the pull chamber16.

[0041] According to the process of the present invention, thetemperature of the ingot I is maintained above the temperature T_(A) atwhich intrinsic point defects will become supersaturated and agglomerateduring the growth of the ingot. More specifically, no portion of theingot I is allowed to cool to the temperature T_(A) during the time thecrystal is being grown. Thus unlike the conventional Czochralski processexample illustrated in FIG. 1, the ingot I never passes through theisotherm T_(A) while it is being grown. The restrictions on theproduction of single crystal silicon previously caused by the presenceof the isotherm at T_(A) are removed by the process of the presentinvention. It is envisioned that control of the cooling of the ingot Imay be achieved by thermal shielding, the application of heat or somecombination of the two. In the illustrated embodiment, the pull chamber16 is provided with a heater 30 (schematically shown in FIG. 3) forapplying heat to the ingot I as it nears and enters the pull chamber.

[0042] The ingot I is held at temperatures above T_(A) for a period oftime selected to permit out-diffusion of intrinsic point defects to aconcentration within the ingot below a solubility limit necessary foragglomeration of intrinsic point defects to occur. The times (discussedmore fully below) necessary for out-diffusion of intrinsic point defectsare generally significantly longer than the normal cycle time of thecrystal puller 10. To that end, the process of the present inventionfurther comprises removing the ingot I from the location of the crystalpuller 10 within a semiconductor production facility to permit thecrystal puller to be recycled independently of the thermal condition ofthe ingot. The ingot I is maintained at temperatures above T_(A) duringand after the time it is removed from the crystal puller 10.

[0043] In the embodiment shown in FIG. 3, removal of the ingot I fromthe crystal puller 10 includes pulling the grown crystal entirely withinthe pull chamber 16. The valve 24 on the pull chamber 16 and the valve26 on the growth chamber 12 are closed, isolating the chambers from eachother and from the surrounding environment. The pull chamber 16 is thendetached from the growth chamber 12 and moved away, as illustrated bythe position of the other pull chamber 16′ in FIG. 3. The pull chamber16 holds the ingot I at the temperature above T_(A) until such time assufficient out-diffusion of intrinsic point defects has occurred. Theingot I can then be cooled to ambient and removed from the pull chamber16 for further processing into semiconductor wafers.

[0044] The heater in the growth chamber 12 can be deactivated so thatthe growth chamber may cool to ambient. The growth chamber 12 is thenopened up so that the crucible 14 can be removed and replaced withanother crystal. Solid polysilicon held in the crucible 14 is melted toform a new melt. At the appropriate time after removal of the pullchamber 16, the other pull chamber 16′ (first having had the ingot I′held therein removed) is moved into place on the growth chamber 12. Thepull chamber 16′ is attached to the growth chamber 12 and the valves24′, 26 of the pull chamber and growth chamber are opened to permitanother single crystal silicon ingot I′ to be grown.

[0045] The total time necessary to hold the ingot I above T_(A) dependsupon the initial concentration of intrinsic point defects, the type ofintrinsic point defects which predominate in the ingot and the diameterof the ingot being grown. Referring now to FIG. 2, the concentration ofboth types of intrinsic point defects is plotted against the ratio ofthe pull rate v over the average instantaneous axial temperaturegradient G₀ in the ingot I at the surface of the melt. At lower ratiosof v/G₀, self-interstitial intrinsic point defects [I] predominate, andat higher ratios vacancy defects [V] predominate. However, it may beseen that at a critical ratio of v/G₀ concentrations of both types ofintrinsic point defects are minimized. Presently, it is believed thatthis ratio is about 2.1×10⁻⁵ cm²/sK. It is desirable to maintain theratio v/G₀ near the critical value, but this is difficult to do over thecourse of the entire growth process of the ingot I, particularly at theseed-cone and end-cone ends. A feature of the present invention that thegrowth of the ingot I is somewhat less dependent upon the v/G₀ ratiobecause of the out-diffusion of intrinsic point defects permitted by thepresent invention without significantly affecting the cycle time of thecrystal puller 10.

[0046] Preferably, the self-interstitial type of intrinsic point defectwill predominate in the ingot I grown according to the process of thepresent invention. Self-interstitial defects [I] are substantially moremobile than vacancy defects [V]. Radial out-diffusion ofself-interstitials can occur about 10 times faster than out-diffusion ofvacancies. Stated another way, it takes ten times as long to out-diffusevacancies in a vacancy dominated ingot as to out-diffuse interstitialsof the same concentration in an interstitial dominated ingot. As aresult, it is preferred that the ratio v/G₀ be maintained below thecritical value over substantial portions of the growth of the ingot I sothat self-interstitial defects will predominate. Of course, v/G₀ variesacross the radius of the ingot I so that there will be a radialvariation of concentration and type of defect within the ingot. However,movement into the vacancy dominated region of v/G₀ for portions of theingot I is permitted so long as self-interstitial defects predominatesufficiently to recombine with the vacancies during out-diffusion,thereby annihilating both defects so that their concentration remainsbelow the solubility limit.

[0047] It is to be understood, however, that the process of the presentinvention may be used for vacancy dominated material. As a general rule,vacancy dominated material (if present at all) will be present at theaxial center of the ingot and, depending upon crystal growth conditions,may extend from the center to the edge of the ingot. In those instancesin which vacancy dominated material does not extend from center to edge,a core of vacancy dominated material will be surrounded by an annularring of interstitial dominated material. Due to the slower mobility ofvacancies in the silicon lattice (as compared to siliconself-interstitial atoms), the times involved to achieve the necessaryrelaxation of the vacancy system (i.e., suppression of the vacancyconcentration) by out-diffusion of vacancies to the surface can berelatively long. In one embodiment of the present invention, therefore,the time required to suppress the concentration of vacancies is reducedby injecting silicon self-interstitial atoms into the ingot whichdiffuse to and recombine with the pre-existing vacancies and annihilatethem. In this embodiment, silicon self-interstitial atoms may beinjected by oxidizing the surface of the ingot as it is being held at atemperature in excess of the temperature at which agglomerationreactions occur. Oxidation may be achieved, for example, by exposing theingot to an oxidizing atmosphere (e.g., an atmosphere comprising oxygenor steam, preferably substantially, if not entirely, oxygen or steam)during the holding period. The oxide layer may be grown to a thicknesson the order of several microns (e.g., 3, 4, 5) or even 10 or moremicrons. Because the thickness of the oxide layer affects the rate ofoxidation (which, in turn, affects the rate at which siliconself-interstitial atoms are injected), it may be advantageous in one,two or more cycles to strip the grown oxide layer (for example, withhydrogen or HF vapor), and then reoxidize the crystal surface during theholding period.

[0048] The diameter of the ingot I being grown affects the time neededfor diffusion of intrinsic point defects, simply because the intrinsicpoint defects must traverse greater radial distances as the diameter ofthe ingot increases. The necessary time for diffusion scales with thesquare of the radius of the ingot I. Thus, it has been found that wherethe constant diameter portion of the ingot is about 150 mm, the totaltime at which the ingot resides above T_(A) (i.e., about 1050° C., 1000°C., or even 900° C.) is at least about 10 hours, preferably at leastabout 12 hours, and more preferably at least about 15 hours. The totaltime at which a 200 mm ingot resides in a similar-system at atemperature above T_(A) would thus be at least about 22 hours,preferably at least about 25 hours, and more preferably at least about30 hours, while the total time at which a 300 mm ingot resides in asimilar system at a temperature above T_(A) would thus be at least about48 hours, preferably at least about 60 hours, and more preferably atlest about 75 hours. It is to be understood that the precise times forout-diffusion can be other than described without departing from thescope of the present invention.

[0049] Referring now to FIG. 4, a process of a second embodiment isillustrated. The process is the same as the process of the firstembodiment, except that a pull chamber 116 of the crystal puller 110 isnot removed from a growth chamber 112. In the second embodiment, thepull chamber 116 has been modified to open into a holding chamber 140next to the pull chamber. For purposes of the present description of theprocess, the holding chamber 140 does not form part of the crystalpuller 110 even though it is physically attached to the crystal puller.After the ingot I is fully grown and drawn up into the pull chamber 116,a door (not shown) separating the pull chamber from the holding chamber140 is opened and the ingot is moved into the holding chamber where itis held at temperatures above T_(A) for the appropriate time. A track142 is shown for carrying the ingot I into the holding chamber 140. Inthe illustrated embodiment, the holding chamber 140 has a heater 144.Thereafter, the ingot I is moved into a thermal lock (not shown) topermit cooling and removal of the ingot from the holding chamber 140without compromising the thermal environment of the holding chamber. Inthe meantime, the door separating the pull chamber 116 from the holdingchamber 140 can be closed. Another winch mechanism and pull wire (notshown) are moved into place in the pull chamber 116 for growing anothersingle crystal ingot (not shown).

[0050] A third embodiment of the process of the present invention isshown in FIG. 5. The process is closely similar to that illustrated inFIG. 4 because the pull chamber 216 is not moved, but rather the ingot Iis moved to a holding chamber 240. The primary difference is thatholding chamber 240 is located above rather than to the side of the pullchamber 216. Again, a separate winch mechanism 218′ and pull wire 220′would be moved into place so that growth of another ingot I′ in thecrystal puller 10 can take place without regard to the thermal conditionof the ingot I already grown.

[0051] It is to be further noted that, for each of the above-describedembodiments of the present process wherein the grown ingot is heldwithin the pull chamber to provide sufficient time for out-diffusion, itis preferred that the pull chamber have a non-uniform thermal profile.Stated another way, because at least a portion of the grown ingot hascooled below temperature at which agglomerated intrinsic point defectsform (T_(A)), it is not necessary that this portion be maintained at atemperature in excess of T_(A) upon being removed or separated from thegrowth chamber. In fact, it is preferred that the temperature profilefor this portion of the crystal not exceed T_(A) because if thetemperature is too high (i.e., more than about 1175° C., 1200° C. ormore), the concentration of intrinsic point defects could again beraised above the solubility limit, or critical concentration, as aresult of diffusion. However, while the temperature of this region mustnot be too high, the temperature of the remaining portion of the ingotmust be maintained sufficiently high such that agglomerations do notoccur.

[0052] If a non-uniform thermal profile is employed, preferably thetemperature will gradually increase from the seed end to the tail end,typically ranging from about 1000° C. to about 1200° C. and preferablyranging from about 1050° C. to about 1175° C. The axial locations withinthe ingot having temperatures in excess of T_(A) will then be cooled inaccordance with the present invention, preferably until the temperatureprofile becomes uniform. The ingot may then be further cooled, as iscommon in the art, and removed for additional processing.

[0053] While a non-uniform temperature profile is preferred, it is to benoted that a uniform temperature profile may also be employed. However,if a uniform profile is to be used, the temperature must be sufficientlyabove T_(A) to prevent agglomeration from occurring, but not so highthat regions which have previously been cooled below T_(A) are againcapable of becoming critical supersaturated (as discussed above).Accordingly, therefore, the temperature will preferably range from about1125° C. to about 1200° C., and more preferably from about 1150° C. toabout 1175° C. Once the ingot is inside the chamber, the temperature ofthis uniform profile will then be reduced in accordance with the presentinvention to cool the ingot below T_(A). The ingot may then be furthercooled, as is common in the art, and removed for additional processing.

[0054] The following Example illustrates the invention.

EXAMPLE

[0055] Two 200 mm crystal ingots were grown in a crystal puller capableof producing fully agglomerated intrinsic defect-free material when theingots are grown at the rate depicted by the dashed line in FIG. 6A(hereinafter, the “defect-free” growth rate curve). The two crystalswere grown at the same target growth rate, depicted in FIG. 6A as acontinuous line, with the growth rate being reported as a normalizedgrowth rate (i.e., the actual growth rate relative to the criticalgrowth rate, typically expressed as a ratio of the actual growthvelocity over the critical growth velocity). As depicted, the ingotswere initially grown for a period of time at a rate which was in excessof the “defect-free” growth rate curve, then for a period of time at arate which was less than the “defect-free” growth rate curve, and thenagain for a period of time at a rate in excess of the “defect-free”growth rate curve. The first ingot (87GEX) was allowed to cool naturallyin the crystal growth chamber upon completion of the growth of theingot. The second ingot (87GEW), however, was not allowed to coolnaturally in the crystal growth chamber; instead, upon completion of thegrowth of the ingot, the heaters in the hot zone of the crystal pullerremained on and the ingot was held for 30 hours in the pull chamber; thetemperature profile was such that regions of the ingot more than about400 mm from the seed end were held at a temperature in excess of about1,050° C. and regions less than about 400 mm from the seed end were heldat a temperature less than about 1,050° C. during this period.

[0056] The ingots were sliced longitudinally along the central axisrunning parallel to the direction of growth, and then further dividedinto sections, each about 2 mm in thickness. Using a copper decorationtechnique, the sets of longitudinal sections which make-up each ingotfrom seed to tail were intentionally contaminated with copper andheated, the heating conditions being appropriate to dissolve a highconcentration of copper into the sections. Following this heattreatment, the samples were rapidly cooled, during which time the coppereither outdiffused or precipitated at sites where oxide clusters oragglomerated interstitial defects were present. After a standard defectdelineating etch, the samples were visually inspected for the presenceof precipitated impurities; those regions which were free of suchprecipitated impurities corresponded to regions which were free ofagglomerated interstitial defects. Photographs were then taken of thesections of each crystal and the photographs assembled to show theresults for each crystal from seed to tail end. The set of photographsfor the first, naturally-cooled ingot (87GEX) are depicted in FIG. 6Band the set of photographs for the second, held ingot (87GEW) aredepicted in FIG. 6C.

[0057] Referring now to FIGS. 6A, 6B, and 6C, it can be seen that thenaturally-cooled ingot (87GEX) contains agglomerated vacancy defectsfrom 0 to about 393 mm, no agglomerated intrinsic point defects fromabout 393 mm to about 435 mm, agglomerated intrinsic point defects fromabout 435 mm to about 513 mm, no agglomerated intrinsic point defectsfrom about 513 mm to about 557 mm, and agglomerated vacancy defects from557 mm to the end of the crystal. These correspond to the regions above,within and below the defect-free growth conditions for this hot zone.The held ingot (87GEW) contains agglomerated vacancy defects from 0 toabout 395 mm, no agglomerated intrinsic point defects from about 395 mmto about 584 mm, and agglomerated vacancy defects from about 584 mm tothe end of the crystal. The most significant difference between the twoingots, therefore, occurs in the region from about 435 mm to about 513mm in which the naturally-cooled ingot (87GEX) contains agglomeratedintrinsic point defects whereas the held ingot (87GEW) does not. Duringthe holding period, the concentration of self-interstitial silicon atomsin the held ingot (87GEW) is suppressed by additional diffusion of theself-interstitial atoms to the surface of the ingot and vacancydominated regions and thus, critical supersaturation and theagglomeration reaction for interstitial atoms is avoided subsequent tocrystal solidification. In the naturally cooled ingot, however,insufficient time is allowed for additional diffusion to the surface andvacancy dominated regions and, as a result, the system becomescritically supersaturated in silicon self-interstitial atoms and anagglomeration reaction occurs.

[0058] These ingots thus illustrate that given sufficient amounts oftime and sufficiently high temperatures, virtually any amount of siliconself-interstitial atoms can be outdiffused to the surface.

[0059] In addition, the “defect-free” growth rate curve depicted in FIG.6A falls within a range of crystal growth rates which provide fullyagglomerated intrinsic defect-free material under natural coolingconditions for this crystal puller configuration. Even under naturalcooling conditions for this hot zone configuration, there is a range ofcrystal growth rates between the growth rate (P_(v)) at whichagglomerated vacancy defects form and the growth rate (P_(I)) at whichagglomerated intrinsic point defects form; this range is at least ±5% ofthe average of P_(v), and P_(I). When the residence time of the growncrystal at temperatures in excess of about 1,050° C. is increased, thisrange is increased further with the range being, for example, at least±7.5%, at least ±10%, or even at least ±15% of the average of P_(v) andP_(I) (for example, for crystal 87GEW the residence was sufficientlygreat that, P_(I), was not achieved and thus, P_(I) for this crystal wasless than the lowest pull rate achieved). These results are presented inTable I, below. TABLE I Tran- position Normalized Window % variationsition mm Pull rate, V V_ave (DV) 100 (DV/Vave) 87GEX V-P 393 0.251 P-I435 0.213 0.232017 0.03865546 16.66 I-P 513 0.209 P-V 557 0.249 0.229370.0402521 17.55 87GEW V-P 395 0.246 P* 465 0.196 0.221008 0.0504201722.81 P* 465 0.196 P-V 584 0.271 0.233193 0.07478992 32.07

[0060] The increase in window size (or allowable pull rate variation fordefect-free growth) is substantially limited to slower pull rates (i.e.,to values smaller than the critical v/G for vacancy to interstitialdominated material (plus some small delta to take into accountinterstitial annihilation of vacancies)). That is to say that the effectis strongest for interstitial dominated material, siliconself-interstitial atoms being a faster diffusing element than vacancies.In other words, the window opens more quickly toward lower pull rates.In principle, the window for allowable pull rate variation towardsfaster pull rates (greater than the critical v/G value plus some smalldelta) into vacancy dominated material) would also open towards fasterpull rates (vacancy dominated material) with increased dwell times attemperatures greater than about 1050° C.—as vacancies diffuse toward thecrystal surfaces—but this would require significantly longer times.

[0061] For a given crystal puller and hot zone configuration, it may beassumed that the axial temperature gradient, G₀ is approximatelyconstant over relatively short distances such as occur in the transitionranges which occur here. As a consequence, a change in the crystalgrowth rate leads to a proportional change in v/G₀ and thus, the initialconcentration of vacancies and silicon self-interstitial atoms. Ingeneral, however, the value of v/G₀ at the center ingot is the mostcritical value since it is the farthest distance from the surface. Thus,the results of this example demonstrate that the increase in pull ratevariations achieved through increased dwell times at temperaturesgreater than about 1000° C. implies that corresponding variations inv/G₀ may occur at any point along the radius of the crystal. In otherwords, radial variation of v/G₀ is irrelevant and thus, for example, mayexceed (at any radial position) 10%, 15% or more of the value of v/G₀ atthe center of the ingot.

[0062] In view of the above, it will be seen that the several objects ofthe invention are achieved.

[0063] As various changes could be made in the above compositions andprocesses without departing from the scope of the invention, it isintended that all matter contained in the above description beinterpreted as illustrative and not in a limiting sense.

What is claimed is:
 1. A process of producing a single crystal siliconingot having a seed-cone, and end-cone and a constant diameter portionbetween the seed-cone and end-cone, the ingot being grown from a siliconmelt in accordance with the Czochralski method, the process comprisinggrowing the ingot from the silicon melt such that the ingot comprises aregion which is dominated by vacancy-type intrinsic point defects andcontrolling the temperature of the ingot such that no portion of theingot cools to a temperature less than a temperature T_(A) at whichagglomeration of intrinsic point defects in the ingot occurs during thetime the ingot is being grown such that at least the constant diameterportion of the ingot is substantially free of agglomerated intrinsicpoint defects.
 2. A process as set forth in claim 1 wherein the ingot ismaintained at temperatures above T_(A) for a period of time selected topermit out diffusion of intrinsic point defects to achieve aconcentration below a solubility limit required for agglomeration ofintrinsic point defects to occur.
 3. A process as set forth in claim 1wherein the ingot is exposed to an oxidizing atmosphere as the ingot ismaintained at a temperature in excess of the temperature T_(A) at whichagglomeration of intrinsic point defects in the ingot occurs.
 4. Aprocess as set forth in claim 3 wherein the ingot is exposed to at leastone cycle as the ingot is maintained at a temperature in excess of thetemperature T_(A) at which agglomeration of intrinsic point defects inthe ingot occurs wherein in the first phase of the cycle the ingot isexposed to an oxidizing atmosphere and in the second phase of the cyclethe ingot is exposed to an atmosphere which dissolves or otherwiseremoves silicon dioxide from the surface of the ingot.
 5. A process ofproducing a single crystal silicon ingot having a seed-cone, andend-cone and a constant diameter portion between the seed-cone andend-cone, the ingot being grown in a crystal puller from a silicon meltin accordance with the Czochralski method, the crystal puller includinga lower growth chamber and an upper pulling chamber, the processcomprising: lowering a seed crystal into contact with the silicon meltlocated in the growth chamber of the crystal puller; withdrawing theseed crystal from the melt so as to cause silicon from the melt tofreeze for forming the single crystal silicon ingot; pulling the fullyformed ingot into the pulling chamber, wherein the fully formed ingotcomprises a region which is dominated by vacancy-type intrinsic pointdefects; isolating the pulling chamber from the growth chamber;maintaining the temperature in the pulling chamber above a temperatureT_(A) at which agglomeration of intrinsic point defects in the ingotoccurs.
 6. A process as set forth in claim 5 wherein the ingot ismaintained at temperatures above T_(A) for a period of time selected topermit out diffusion of intrinsic point defects to achieve aconcentration below a solubility limit required for agglomeration ofintrinsic point defects to occur.
 7. A process as set forth in claim 5wherein the ingot is exposed to an oxidizing atmosphere as the ingot ismaintained at a temperature in excess of the temperature T_(A) at whichagglomeration of intrinsic point defects in the ingot occurs.
 8. Aprocess as set forth in claim 7 wherein the, ingot is exposed to atleast one cycle as the ingot is maintained at a temperature in excess ofthe temperature T_(A) at which agglomeration of intrinsic point defectsin the ingot occurs wherein in the first phase of the cycle the ingot isexposed to an oxidizing atmosphere and in the second phase of the cyclethe ingot is exposed to an atmosphere which dissolves or otherwiseremoves silicon dioxide from the surface of the ingot.
 9. A process forgrowing a single crystal silicon ingot, the ingot being grown from asilicon melt in accordance with the Czochralski method in a crystalpuller comprising a lower growth chamber and an upper pulling chamber,the process comprising: growing the silicon ingot in the growth chambersuch that the ingot comprises a region which is dominated byvacancy-type intrinsic point defects; cooling a first portion of theingot to a temperature less than a temperature of agglomeration, T_(A),while maintaining a second portion of the ingot at a temperature greaterthan T_(A) during the growth of the ingot, wherein the temperature ofagglomeration is the temperature at which the agglomeration of intrinsicpoint defects occurs; transferring the grown ingot to the pullingchamber; and, maintaining a temperature profile in the pulling chambersuch that the temperature of the cooled first portion of the ingot doesnot exceed about 1200□C. and the second portion of the ingot ismaintained at a temperature greater than T_(A) for a period of timeselected to permit out diffusion of intrinsic point defects to reducethe concentration of intrinsic point defects such that the secondportion of the ingot is substantially free of agglomerated intrinsicpoint defects upon being cooled to a temperature less than T_(A).
 10. Aprocess as set forth in claim 9 wherein the ingot is exposed to anoxidizing atmosphere as the second portion of the ingot is maintained ata temperature in excess of the temperature T_(A).
 11. A process as setforth in claim 10 wherein the ingot is exposed to at least one cycle, asthe second portion of the ingot is maintained at a temperature in excessof the temperature T_(A), wherein in the first phase of the cycle theingot is exposed to an oxidizing atmosphere and in the second phase ofthe cycle the ingot is exposed to an atmosphere which dissolves orotherwise removes silicon dioxide from the surface of the ingot.